Ultrasound assisted crystallisation in microreactors

This research project studied the use of ultrasound in cooling crystallization as an alternative seeding and particle engineering technique. To accomplish this goal, the fundamentals of the cavitation phenomena were first studied to identify the operating conditions suitable for crystallization. Next, the implementation, operation and settings of the ultrasonic field during cooling crystallization were optimized. Finally, possible reactor designs and scale-up strategies were examined for the reactive crystallization of manganese carbonate.

This research demonstrates that a multibubble sonoluminescence technique can be used to distinguish between stable and transient bubbles which exhibit different characteristics. The generated type is related to both ultrasonic, operating and design parameters. The chemical reactivity of both bubble types was quantified in the presence of various gases and it was shown that this strongly correlated to the sonoluminescence signal. These results indicate that operating with transient bubbles at low frequency or addition of gases, which reduce the bubble core temperature, minimizes the chemical degradation and thus provides suitable operating conditions within crystallization. In this suitable process window for ultrasound, the power consumption during ultrasound-assisted cooling crystallization can be reduced by the use of pulsed sonication. If the pulse-off time of the pulsed ultrasonic field is equal or shorter than the bubble dissolution time, a similar improvement in nucleation can be obtained as under continuous sonication. Further variation of the on- and off-period allows to control the particle size, without any significant effect on the particle shape. Later on, follow-up research on particle size and shape control identified that the particle size and degree of agglomeration can be best controlled by application of ultrasound immediately after the first appearance of nuclei. Enhanced secondary nucleation, crystal breakage and disaggregation by a higher collision frequency were identified as the main mechanism for particle size and agglomeration control. Finally, some ultrasonic reactor configurations were considered for a reactive crystallization process in which the spherical particles needed to be produced. These were assessed using earlier developed characterization techniques that map the ultrasonic energy distribution, and the mechanistic model for particle size and shape control which identified the required ultrasonic effects. This evaluation provides some evidence that scale-up is feasible and showed that recirculation loops in which the ultrasonic energy is confined in a small volume can be easily implemented on existing facilities. However, further optimization and additional knowledge on scale-up rules are needed for industrial implementation.